In this investigation nine models of circular crested weir were constructed and tested during an experimental study. Crest radius( r) of the weir was varied three times; 2.5 cm, 3.1cm and 3.75cm. For every crest radius the weir height (p) was varied three times; 15cm, 20cm and 25cm. The result was showed that the discharge coefficient (Cd) affected by two factors; head above the weir to the crest height (h/p) and head above the weir to the crest radius (h/r). It was observed that coefficient of discharge (Cd) increased with increasing (h/p) for the similar height of weir and for definite value for the ratio h/p the (Cd) value increased with increasing the crest height (p). Also it was found that coefficient of discharge (Cd) increased with increasing (h/r) for the similar crest radius and for definite value for the ratio (h/r), the (Cd) value increased with increasing the crest radius (r). A simple power experiential expression was found for the variation of (Cd) with (h/p) and (h/r) with high correlation coefficient.